Micromachined sensor with insulating protection of connections

ABSTRACT

The invention relates to sensors of physical quantities such as pressure or acceleration sensors and, more specifically, to the mounting of the active part of the sensor on a base ( 30 ) bearing connection pins ( 32 ).  
     According to the invention, an active part of the sensor is prepared. This active part is formed, for example, by micro-machined silicon wafers ( 10, 12 ) bearing electronic elements, electrical conductors and connection pads ( 22 ). A base ( 30 ) is thus prepared, provided with pins ( 32 ), and the pads ( 22 ) are electrically connected to the pin ends by conductive elements (wires  40 ). Then the wafer and the pin ends are plunged into an electrolytic bath so as to make an electrolytic deposit of conductive metal ( 42 ) on the pin ends, the pads and the conductive elements that connect them. Finally, this metal is oxidized or nitrized to constitute an insulating coat ( 44 ) on the connection pin ends, the pads and the conductive elements that connect them.  
     Application to sensors of pressure, stresses, acceleration etc, designed to work in harsh environments.

[0001] The invention relates to the mounting of sensors of physicalquantities capable of working in harsh environments.

[0002] The mounting generally consists of the transfer of amicro-machined sensor to a base provided with electrical connectionpins. The sensor is made, for example, out of several machined siliconwafers comprising mechanical elements (diaphragms, beams, seismicmasses, etc), electronic elements (capacitor plates or strain gauges inparticular), and metal contact pads used for electrical connection withthe pins of the base when the sensor is fixed to the base.

[0003] Classically, the sensor is bonded or brazed by its rear face tothe base, in a central part of this base that is surrounded byconnection pins going through the base. The connection pads of thesensor, on the front face of this sensor, are connected by bonded wiresbetween the connection pads and the tips of the connection pins thatemerge from the surface of the base.

[0004] In this case, to ensure efficient operation in a harsh, wet orgaseous environment, it is necessary to cover the bonded wires, theconnection pads and the ends of the pins with with a protectiveinsulating layer that prevents, firstly, the impairment of the sensorand, secondly, leakage currents between pins when the liquid or gasenvironment is not perfectly insulating. These leakage resistors indeeddisturb the measurement of physical quantities which often relies onvery small differential variations in resistance or on very weakelectrical signals.

[0005] A polymerizable material, such as silicone resin, is thendeposited on the conductive parts. Parylene may also be deposited.However, in applications such as pressure sensors, comprising a thindiaphragm in contact with the medium whose pressure is to be measured,it is necessary to avoid depositing this material on the diaphragm,because this would give rise to measurement errors and it is difficultto be aware of these errors and compensate for. Special precautionstherefore have to be taken in depositing operations, and it may even benecessary to work by hand. Furthermore, this type of coating does notalways withstand harsh environments.

[0006] The invention proposes to carry out an electrolytic deposition ofmetal on the conductive parts, followed by an operation for theoxidizing or nitriding of this metal so as to achieve the coating, witha layer of insulating oxide or nitride, of all the conductive parts thatmay subsequently come into contact with an ambient medium that is notperfectly insulating.

[0007] More specifically, the invention proposes a method for making asensor of physical quantities consisting of the preparation of an activesensor part and a base, the active part comprising at least one waferprovided with conductive connection pads on one face and the base beingprovided with conductive pins, the electrical connection of the pads andthe pins by conductive elements and then the plunging of the wafer andthe pin ends into an electrolytic bath, the performance of anelectrolytic deposition of at least one conductive metal on the pinends, the pads and the conductive elements that connect them and theperformance of an oxidizing or nitrizing operation on this metal to makean insulating coating on the connection pads, the connection pin endsand the conductive elements that connect them. The electrolytic depositis made not only on the conductive parts but also on the insulatingparts.

[0008] The term “electrolytic deposit” is understood to mean a metaldeposit (a single metal or an alloy or combination of metals depositedsimultaneously or successively) on a conductive zone obtained by themigration of metal ions coming from a liquid solution. The migration maybe prompted either by the passage of an electrical current (a classicelectrolytic bath with current lead-in electrodes), or by chemicalreaction (using what is called electroless deposition).

[0009] This method may be implemented either when the bonding wires arebonded between the pads and the pin ends or when the pin ends are eachsoldered directly to a respective pad.

[0010] The electrolytic deposit designed to be then oxidised or nitrizedmay be be, for example, a tantalum deposit giving rise to a tantalumoxide or tantalum nitride coating that is especially resistant tochemical corrosion or to temperature and pressure.

[0011] The oxidizing will generally be carried out in a step subsequentto the step of electrolytic metal deposition, but it is sometimespossible to obtain the metal oxide directly during the electrolysisitself rather than to carry out a metal deposition and then an oxidizingoperation in succession.

[0012] Other features and advantages of the invention shall appear fromthe following description made with reference to the appended drawings,of which:

[0013]FIG. 1 shows a sensor whose active part is connected by bondedwires to the pins of the base;

[0014]FIG. 2 shows the sensor of FIG. 1, after electrolytic metaldeposition on the bonded wires and on the connection pads;

[0015]FIG. 3 shows the sensor according to the invention, after surfaceoxidising of the electrolytic deposit;

[0016]FIG. 4 shows a sensor whose active part is mounted in an invertedposition and soldered to a base by an operation of electrolytic metaldeposition;

[0017]FIG. 5 shows the sensor of FIG. 4 after electrolytic deposition ofthe second metal layer and after the surface oxidising of this secondmetal layer.

[0018] The invention shall be described with reference to a pressuresensor that has to work in a harsh environment, for example a sensorused to gauge the pressure of exhaust gases from an internal combustionengine or a pressure sensor placed within the cylinder of such anengine. The environment therein is harsh because of the very hightemperatures (several hundreds of degrees Celsius) and the noxiousnessof the surrounding environment (in terms of corrosive gases).

[0019] The invention can be applied however to other sensors.

[0020]FIG. 1 shows the sensor in an intermediate stage of manufacture,in which the active part of the sensor has been bonded to a base andconnection wires have been bonded between connection pads of the activepart and connection pins mounted on the base.

[0021] The active part of the sensor, in this example, is made out oftwo soldered silicon wafers 10 and 12, machined so as to demarcate acavity 14 closed by a thin silicon diaphragm 16. The wafer 10 could bemade of glass.

[0022] On the diaphragm 6, electronic elements 18 needed to detect thedeformations of this diaphragm are formed by means of microelectronicmanufacturing methods. In one example, these elements are strain gaugesdirectly formed in the silicon (by the implantation of appropriatedopants in the silicon) or formed in a silicon layer separated from thesilicon substrate by an insulating layer (this is thesilicon-on-insulator structure). For very harsh environments, thesegauges may be made on the diaphragm inside the cavity 14. If theenvironment is less difficult, they may be formed outside the cavity 14.The gauges are sensitive to the the deformations of the diaphragmprompted by the pressure variations to be measured.

[0023] Electrical connections 20 used for the power supply of the gaugesand for the transmission of measurements made on these gauges are formedon the active part of the sensor. On a front face of the active part ofthe sensor, these connections lead to connection pads 22 which areconductive metal surfaces used for the electrical connection withexternal pins. The front face or main face of the active part of thesensor is the one facing upwards in FIG. 1. The front face is generallyprotected by a passivation layer 24 (made of silicon oxide or nitridefor example) that lines the entire surface except for the connectionpads 22 or at least their central part.

[0024] For the mounting of the active part of the sensor on a base, abase 30 is made with metal connection pins 32 going through this base.The number of these pins 32 is equal to the number of connection padspresent on the sensor and necessary for the working of the sensor. Theupper part of the pins reaches the upper surface or above the uppersurface of the base. The lower part descends beneath the lower surfaceof the base and can be plugged for example into a female connector orinto one of the holes of a printed circuit, or bonded to individualconductive wires, etc.

[0025] The base may be any insulator or conductive base, but in thelatter case it must be planned that an insulator 33 (for example made ofglass in the case of a metal base) will fill the passages into which thepins are inserted, in order to electrically insulate the pins from oneanother. In one embodiment, the base is a metal alloy such as Kovar,with glass-lined via holes. It could be made of insulating ceramic oreven plastic for environments at moderate temperatures.

[0026] The active part of the sensor is soldered by its rear face to theupper surface of the base.

[0027] Conductive connection wires (for example gold wires) are bondedbetween the connection pads 22 and tips of the pins 32.

[0028] In the method according to the invention, the active part of thesensor as well as the upper part of the pins are then plunged into anelectrolytic bath so that a conductive metal deposit is formed, byelectrolytic migration, on the pads 22, the wires 40 and the upper partof the pins 32. The electrolytic deposit is formed only on theconductive parts plunged into the bath. In particular, it does not formon the diaphragm 16 lined with the passivation layer 24, so that themechanical characteristics of the diaphragm are not impaired by theelectrolytic deposit. One or more metals may be deposited, especially inthe form of an alloy, or several metals may be deposited simultaneously.

[0029]FIG. 2 shows the sensor thus lined with an electrolytic deposit 42on all its conductive parts above the base: the parts located below thebase are not plunged into the bath.

[0030] The metal deposited by electrolysis may be for example tantalum,but other metals are possible, especially nickel or tungsten ormolybdenum. A combination of metals (alloy or co-deposition) may also beconsidered. The connection pads may be made of gold or other metals or acombination of metals (sometimes with several metal layerssuperimposed). If the deposit is made by classic electrolysis with thepassage of current into a solution containing metal ions, it is seen toit that all the pins are connected together during the time of theelectrolysis (preferably through the rear of the base, namely through apart that is not plunged into the electrolytic bath). An appropriatedifference in electrolysis potential is applied between these pins andanother electrode plunged into the bath.

[0031] And electroless deposit is also possible. In this case, theelectrolysis occurs by simple chemical reaction between the pins areconnection pads and the ion solution of the electrolytic bath, withoutthe application of external potential differences.

[0032] An operation of surface oxidizing or nitrizing is then carriedout on the electrolytic layer 42. The oxidized or nitrized surface layer44 thus formed (FIG. 3) is insulating and highly resistant to corrosionby the external environment. In particular, the tantalum oxide thatforms when the layer 42 is made of tantalum is highly resistant, even athigh temperature, to moisture penetration, air salinity, corrosiveagents etc. The conductive parts located above the base that had beenlined with the electrolytic deposit 42 are thus lined with theinsulating protective layer 44.

[0033] Apart from the fact that it protects the conductive parts againstthe corrosive environment, the layer of protective insulator 44 has theadvantage of removing the need to protect the sensor by means of aninsulating oil bath and a metal diaphragm as was done sometimes in theprior art to prevent electrical leakage between pins carried todifferent potentials. This type of assembly was costly, and the presenceof the oil bath modified the characteristics proper to the sensor: forexample, in the case of a pressure sensor, the external pressure wastransmitted through the oil bath, thus generating measurement errorsdifficult to compensate for.

[0034]FIG. 3 shows the sensor provided with the layer 44 on all theconductive parts located above the base.

[0035] The parts of pins emerging out of the rear of the base areprotected during the oxidizing or nitrizing operation, or else they arecleaned after this operation.

[0036] The oxidizing or nitrizing of the electrolytic layer depositedcan be done either by annealing in an oxidizing atmosphere or by dippingin a chemical bath or an oxidising electrolytic bath. Sometimes, it caneven be done during the electroless deposition.

[0037] The invention can be applied in another configuration, when theactive part of the sensor is inverted with its front face pointingdownwards, namely pointing toward the base, the active part beingdirectly soldered by its conductive pads 22 to the tips of the pins 32.

[0038]FIG. 4 shows an intermediate manufacturing step in which theactive part of the sensor has been fixed to its base as follows: eachpin tip 32 reaches beyond the upper surface of the base, facing arespective pad 22, and it is held against this pad while the entireactive part of the sensor and tips of the pads are plunged into anelectrolytic bath. A metal deposit 34 is formed both on the pads and thepin ends. This deposit forms an electrolytic solder between the pads andthe pins and the electrolysis is continued for a period of time longenough for the thickness deposited to form a rigid mechanical bondbetween the pads and the pins. The conductive elements between the pads22 and the tips of the pins are constituted, in this case, by theelectrolytic deposit 34 and not by wires as in FIGS. 1 to 3.

[0039] If the metal thus deposited by electrolytic means can be easilyoxidized or nitrized, and if the oxide or nitride layer thus formed hasthe desired characteristics of resistance to corrosion, then the surfaceoxidizing or nitrizing of this metal can be done directly, either byannealing in an oxidising atmosphere or by dipping in an oxidising bathto make the desired protection layer on the conductive elements. If, onthe contrary, the deposited metal is not easy to oxidize or to nitrize,or if the oxide or nitride formed is not resistant enough in theenvironment envisaged, then a new electrolytic deposit of another metal(tantalum especially) is made, followed by the surface oxidizing ornitrizing of this metal. This is the case especially if the firstelectrolytic deposit, used to solder the pads to the pins, is a copperdeposit.

[0040]FIG. 5 shows the sensor thus covered with the firstelectrolytically deposited layer 34 (copper for example) and then asecond electrolytic layer 35 (preferably tantalum) and finally theinsulating, oxidized surface layer 36 (tantalum oxide Ta₂O₅).

[0041] The invention can be applied especially to sensors of pressure,stresses, acceleration, temperature, gas or liquid, working in harshenvironments.

1. Method for making a sensor of physical quantities consisting of thepreparation of an active sensor part (10, 12) and a base (30), theactive part comprising at least one wafer provided with conductiveconnection pads (22) on one face and the base being provided withconductive pins (32), the electrical connection of the pads and the pinsby conductive elements (40) and then the plunging of the wafer and thepin ends into an electrolytic bath, the performance of an electrolyticdeposition of at least one conductive metal (42) on the pin ends, thepads and the conductive elements that connect them and the performanceof an oxidizing or nitrizing operation on this metal to make aninsulating coat (44) on the connection pads, the connection pin ends andthe conductive elements that connect them.
 2. Method according to claim1, characterized in that the electrolytic deposition is obtained by themigration of metal ions coming from a liquid solution, with the passageof electrical current into the solution.
 3. Method according to claim 1,characterized in that the electrolytic deposition is an electrolessdeposition carried out by the migration of metal ions coming from aliquid solution, without the passage of electrical current.
 4. Methodaccording to one of the above claims, characterized in that theelectrolytically deposited conductive metal is nickel or tantalum ortungsten or molybdenum.
 5. Method according to one of the claims 1 to 4,characterized in that the connection pins and the connection pads areconnected by bonded wires (40).
 6. Method according to one of the claims1 to 4, characterized in that the conductive elements that connect thepads electrically and mechanically to the pins are constituted by anelectrolytic metal deposit (34).
 7. Sensor of physical quantitiesobtained by the method of one of the above claims, characterized in thatit constitutes a sensor of pressure, stresses, acceleration,temperature, gas or liquid.